Problem: 4 people can paint 3 walls in 45 minutes. How many minutes will it take for 7 people to paint 9 walls? Round to the nearest minute.
Answer: We know the following about the number of walls $w$ painted by $p$ people in $t$ minutes at a constant rate $r$ $w = r \cdot t \cdot p$ $\begin{align*}w &= 3\text{ walls}\\ p &= 4\text{ people}\\ t &= 45\text{ minutes}\end{align*}$ Substituting known values and solving for $r$ $r = \dfrac{w}{t \cdot p}= \dfrac{3}{45 \cdot 4} = \dfrac{1}{60}\text{ walls painted per minute per person}$ We can now calculate the amount of time to paint 9 walls with 7 people. $t = \dfrac{w}{r \cdot p} = \dfrac{9}{\dfrac{1}{60} \cdot 7} = \dfrac{9}{\dfrac{7}{60}} = \dfrac{540}{7}\text{ minutes}$ $= 77 \dfrac{1}{7}\text{ minutes}$ Round to the nearest minute: $t = 77\text{ minutes}$